Apparatus and method for controlling the flow of a fluid

ABSTRACT

An apparatus for controlling the flow of a fluid includes a LP fluid inlet ( 18 ) and a LP fluid outlet ( 9 ) connected to a flare/vent system. A LP flow line ( 20 ) connects the LP fluid inlet ( 18 ) to the LP inlet  6   a  of a jet pump ( 6 ). A supply of HP fluid is connected to jet pump inlet ( 6   b ), and a MP line ( 8 ) is connected to jet pump outlet ( 6   c ). A control unit ( 28 ) is configured to control operation of first and second LP flow control valves ( 4, 3 ) according to signals received from a flow meter ( 12 ), so that LP gas is diverted from the flare system to the LP inlet ( 6   a ) of the jet pump for pressurisation.

The present invention relates to an apparatus and a method for controlling the flow of a fluid, and in particular but not exclusively to an apparatus and a method for controlling the flow of a fluid to avoid flaring or venting of low pressure (LP) hydrocarbon gas.

The flare systems provided on both onshore and offshore oil and gas production installations enable low pressure (LP) gas to be flared whenever needed. The main reason for flaring LP gas is that operators find it uneconomical to use a dedicated compressor to boost the pressure of the LP gas to a higher pressure so that it can be utilised as fuel gas or for other useful purposes.

Although the present invention in concerned mainly with LP hydrocarbon gas, the flow of gas may have droplets of liquid (oil or water) entrained within it. Also, fluids consisting mainly or wholly of liquids may be used in certain embodiments of the invention. The term “fluid” as used herein is intended to encompass pure gases, mixtures of gases and liquids and pure liquids.

The terms “low pressure” (LP) and “high pressure” (HP) are used herein in a relative sense, a high pressure being higher than a low pressure. Typically a high pressure fluid will have a pressure at least twice that of a low pressure fluid. For example, a low pressure fluid may typically have a pressure of about 1-2 atmospheres (1-2×10⁵Pa), and a high pressure fluid may typically have a pressure of 2-5 atmospheres (2-5×10⁵Pa) or more.

The LP gas produced in oil and gas production installation may suffer from the following conditions:

-   -   Its flow rate is small, often no more than a few MMscfd (million         standard cubic feet per day) of gas at near atmospheric pressure     -   The flow rate is often erratic and may vary depending on the         temperature of the surrounding environment, other process system         upset conditions, variations to production from the wells and         other causes.

The small flow rate and unexpected variations in the flow rate of the LP gas make using a compressor difficult and uneconomical. Even if the flow rate of gas is reasonably steady using a compressor can be very costly, as it uses fuel to operate and has high maintenance costs. Also if a compressor is used an extra knock-out pot is needed to remove any liquid droplets carried with the gas before the gas is fed to the compressor. The result is wastage of gas and release of toxic fumes, which are environmentally harmful. In a situation of venting (such as vapour release from storage tanks etc.), often health and safety issues are major causes of concern.

It is an object of the present invention to provide an apparatus and a method for controlling the flow of a fluid that mitigates one or more of the aforesaid problems.

According to one aspect of the present invention there is provided an apparatus for controlling the flow of a fluid, the apparatus including:

-   -   a LP fluid inlet connected to a supply of LP fluid,     -   a LP fluid outlet connected to a flare system for flaring LP         fluid whenever needed,     -   a first LP flow line connecting the LP fluid inlet to the LP         fluid outlet,     -   a jet pump having a LP jet pump inlet, a HP jet pump inlet and a         HP jet pump outlet,     -   a second LP flow line having a first end connected to the first         LP flow line at a first connection point between the LP fluid         inlet and the LP fluid outlet and a second end connected to the         LP jet pump inlet,     -   a HP fluid inlet connected to a supply of HP fluid,     -   a HP flow line having a first end connected to the HP fluid         inlet and a second end connected to the HP jet pump inlet,     -   a medium pressure (MP) jet pump discharge line having a first         end connected to the MP jet pump outlet and a second end         connected to a MP fluid outlet;     -   a first LP flow control valve in the first LP flow line between         the first connection point and the LP fluid outlet,     -   a second LP flow control valve in the second LP flow line         between the first connection point and the LP jet pump inlet,     -   a flow meter configured to detect the flow of LP fluid in the         first LP flow line between the LP fluid inlet and the first         connection point, and     -   a control system connected to the flow meter and to the first         and second LP flow control valves, wherein the control system is         configured to control operation of the first and second LP flow         control valves according to signals received from the flow         meter.

The outlet fluid pressure of the jet pump is always lower than the HP pressure ,but higher than the pressure of LP inlet ,that is why it is referred to as MP (medium pressure) fluid.

The apparatus allows LP gas to be diverted from the flare system and compressed to a pressure that allows it to be put to a useful purpose, such as providing power or heat. Environmental and health and safety problems are thus reduced. A conventional compressor system is not required, thereby reducing the cost of the system. The system can also cope with low and varying flow rates of LP gas.

Preferably, the design of the jet pump allows a full turn-down of the LP flow rate from the design value to a zero flow. In the event of a zero flow rate the LP flow control valves may be closed to prevent sub-atmospheric pressures being established in the LP flow lines.

Alternatively, the jet pump can operate continuously with minimum control if the creation of sub-atmospheric conditions in the LP flow line is not an issue for the operator, or if the flare/vent/relief gas is at a pressure higher than atmospheric pressure.

Advantageously, the control system is configured such that when the flow meter detects a flow of LP fluid, the control system closes the first LP flow control valve on the flare line and opens the second LP flow control valve so that the LP fluid is directed into the LP jet pump inlet.

Preferably the apparatus includes a first HP flow control valve in the HP flow line, said first HP flow control valve being connected to the control system, wherein the control system is configured such that when the flow meter detects a flow of LP fluid the control system opens the first HP flow control valve allowing HP fluid to flow from the HP fluid inlet into the HP inlet of the jet pump. This ensures that the HP motive fluid is only supplied to the jet pump when it is needed.

The apparatus may include a pressure detector for detecting the pressure of fluid in the second LP flow line, said pressure detector being connected to the control system.

Advantageously, the control system is configured to adjust the first HP flow control valve according to the detected pressure of fluid in the second LP flow line, so as to control the flow rate of HP fluid into the HP jet pump inlet. This allows control over the pressure reduction produced at the LP inlet of the jet pump, allowing the pressure at that point to be maintained at a substantially uniform level.

Advantageously, the control system is configured such that when the pressure detector detects a pressure in the second LP flow line that is higher than a predetermined value, the control system opens the first LP control valve so that at least some LP fluid is directed from the LP gas inlet towards the LP gas outlet for temporary flaring. This ensures that if there is a sudden and large increase in the flow of LP gas, which exceeds the capacity of the jet pump, the excess gas can be safely diverted to the flare system.

Advantageously, the apparatus includes:

-   -   a return flow line having a first end connected to the jet pump         discharge line at a second connection point between the HP jet         pump outlet and the HP fluid outlet, and a second end connected         to the second LP flow line between the second LP flow control         valve and the LP jet pump inlet, and     -   a return flow control valve in the return flow line, said         control system being connected to the return flow control valve,     -   wherein the control system is configured to control operation of         the return flow control valve according to signals received from         the pressure detector.

This arrangement allows the jet pump to operate continuously, even when the flow of LP gas is low or absent, the return flow of gas through the return flow line ensuring that the pressure at the LP inlet of the jet pump is not forced too low or below atmospheric pressure.

Advantageously, the control system is switchable to an alternative operational mode, wherein the control system opens the first LP flow control valve and closes the second LP flow control valve so that the LP fluid is directed towards the LP fluid outlet, so directing the LP gas to the flare system.

The apparatus may include a knock-out pot in the first LP flow line between the LP fluid inlet and the first connection point, to catch any droplets of liquid in the LP fluid entering the apparatus.

The apparatus may include a by-pass line connected around the first LP flow control valve and a pressure release device provided within the by-pass line, to allow the safe release of the LP gas to the flare system if the flow rate becomes too high beyond the design capacity of the jet pump.

Advantageously, the apparatus includes an oxygen detector in the first LP flow line and the control system is configured to close the second LP flow control valve if oxygen is detected in the first LP flow line. The control system is thus configured to prevent air being pulled into the system through the flare system in the absence of a flare if for any reason the first LP control valve in the first LP flow line fails to close.

The apparatus may include a plurality of jet pumps connected in parallel, allowing a greater LP flow rate to be handled. The plurality of jet pumps may all be controlled by a single set of flow control valves as described herein.

According to another aspect of the invention there is provided a method for controlling the flow of a fluid in an apparatus according to any one of the preceding statements of invention, the method comprising:

-   -   supplying LP fluid to the LP fluid inlet,     -   supplying HP fluid to the HP fluid inlet,     -   detecting the flow of LP fluid in the first LP flow line between         the LP fluid inlet and the first connection point, and     -   controlling operation of the first and second LP flow control         valves according to signals received from the flow meter.

Advantageously, the method comprises closing the first LP flow control valve and opening the second LP flow control valve when the flow meter detects a flow of LP fluid, to direct LP fluid into the LP jet pump inlet.

Advantageously, the method comprises opening the first HP flow control valve when the flow meter detects a flow of LP fluid, to allow HP fluid to flow from the HP fluid inlet into the HP inlet line of the jet pump.

Advantageously, the method comprises detecting the pressure of fluid in the second LP flow line.

Advantageously, the method includes controlling the flow rate of HP fluid according to the detected pressure of fluid in the second LP flow line.

Advantageously, the method includes opening the first LP control valve to direct at least some LP fluid from the LP gas inlet towards the LP gas outlet and then to the flare system when the pressure detector detects a pressure in the second LP flow line that is higher than a predetermined value.

Advantageously, the method includes controlling operation of the return flow control valve according to signals received from the pressure detector.

Advantageously, the jet pump is operated continuously to capture unexpected flare/vent gas without causing delays in activation of the control system.

Advantageously, the method includes selecting an alternative operational mode comprising opening the first LP flow control valve and closing the second LP flow control valve to direct LP fluid towards the LP fluid outlet.

The solution, according to one embodiment of the invention, involves using a low cost passive device, known as a jet pump (or alternatively as a surface jet pump (SJP), eductor or ejector). Jet pumps use energy from a high pressure (HP) fluid source (which is generally available at an oil and gas production installation) to boost the pressure of LP gas. As a typical performance characteristic, a jet pump can typically boost the pressure of LP gas by up to 4 or 5 times or to a lower boost pressure ratio, depending on the pressure and flow rate of HP gas available as the motive flow. In some cases two jet pump units can operate in series, each raising the pressure of the gas by a factor 2 to 3. In this case the system may for example boost the pressure of LP gas from 1 atmosphere to a pressure that is many times higher than the pressure of the LP gas. In many cases boosting the pressure of LP gas by a factor of 2 to 3 may be sufficient. A plurality of jet pumps can also be installed in parallel to provide a wide pressure and flowrate range for the LP gas.

The proposed solution provided by certain embodiments of the invention also enables the jet pump to cope with variations in the flow rate of LP gas. The variation may range from a zero flow rate to a maximum flow rate that is determined by the operator.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing, wherein:

FIG. 1 is a schematic diagram illustrating the components of an apparatus for controlling the flow of a fluid.

The apparatus includes a LP fluid inlet 18 connected to a supply of LP fluid, a LP fluid outlet 9 connected to a flare system for flaring LP fluid, and a first LP flow line 10 connecting the LP fluid inlet 18 to the LP fluid outlet 9. A jet pump 6 has a LP jet pump inlet 6 a, a HP jet pump inlet 6 b and a HP jet pump outlet 6 c. A second LP flow line 20 has a first end connected to the first LP flow line 10 at a first connection point 25 between the LP fluid inlet 18 and the LP fluid outlet 9, and a second end connected to the LP jet pump inlet 6 a.

A HP fluid inlet 1 is connected to a supply of HP fluid. A HP flow line 26 has a first end connected to the HP fluid inlet 1 and a second end connected to the jet pump inlet 6 b. A jet pump discharge (MP) line 8 has a first end connected to the jet pump outlet 6 c and a second end connected to a MP (medium pressure) fluid outlet 27, which is connected to a downstream gas processing or utilisation system. The outlet fluid pressure of the jet pump is always lower than the HP pressure of line 1 or 6 b, but higher than the pressure of LP inlet line 20: that is why it is referred to as MP (medium pressure) fluid.

A first LP flow control valve 4 is provided in the first LP flow line 10 between the first connection point 25 and the LP fluid outlet 9. A second LP flow control valve 3 is provided in the second LP flow line 20 between the first connection point 25 and the LP jet pump inlet 6 a. A flow meter 12 is located in the first LP flow line 10 and is configured to detect the flow of LP fluid in the first LP flow line between the LP fluid inlet 18 and the first connection point 25.

A control system 28,29 comprising a control unit 28 and a plurality of control lines 29 is connected to the flow meter 12 and to the first and second LP flow control valves 4,3. The control system 28,29 is configured to control operation of the first and second LP flow control valves 4,3 according to signals received from the flow meter 12.

A first HP flow control valve 2 is provided in the HP flow line 26. The first HP flow control valve 2 is connected to the control system 28,29. A pressure detector 11 is connected to the second LP flow line 20 to detect the pressure of fluid in the second LP flow line. The pressure detector 11 is connected to the control unit 28 via a control line 29 and provides the control unit 28 with signals indicating the pressure of fluid in the second LP flow line 20.

A return flow line 7 has a first end connected to the jet pump discharge line 8 at a second connection point 30 between the jet pump outlet 6 c and the MP fluid outlet 27, and a second end connected to the second LP flow line 20 at a third connection point 31 between the second LP flow control valve 3 and the LP jet pump inlet 6 a. A return flow control valve 5 is provided in the return flow line 7. The control system 28,29 is connected to the return flow control valve 5 and is configured to control operation of the return flow control valve 5 according to signals received from the pressure detector 11.

A knock-out pot 17 is provided in the first LP flow line 10 between the LP fluid inlet 18 and the first connection point 25. The knock-out pot 17 is connected to a liquid drain through a drain valve 15 and drain line 16.

A first non-return valve 19 a is provided in the jet pump discharge line 8 and a second non-return valve 19 b is located in the second LP flow line 20 upstream of the LP inlet 6 a of the jet pump 6. Isolation valves 13, 14 a and 14 b are provided in the second LP flow line 20, the HP flow line and the jet pump discharge line 8.

A bypass line 32 is connected in parallel with the first LP flow control valve 4 and a pressure release device 24 equipped for example with a collapsing safety plate is provided within the bypass line 32.

A purge gas line 23 is connected to the first LP flow line 10 at the first connection point 25, the purge gas line being connected to an oxygen detector 22 a. A flow direction meter 21 and an oxygen detector 22 are provided in the first LP flow line 10, between the first connection point 25 and the LP fluid outlet 9.

The apparatus also optionally includes pressure transducers 33,34,35 connected to the first LP flow line upstream and downstream of the flow meter 12 and to the HP flow line 26 upstream of the first HP flow control valve 2.

FIG. 1 illustrates a preferred embodiment of the apparatus, which enables the jet pump 6 to operate safely and efficiently. The apparatus includes a number of control valves and pressure and flow rate instruments, which enable the jet pump 6 to operate whenever needed. They also help the jet pump 6 to handle variations in the LP gas flow rate by adjusting the HP flow rate as needed.

A HP motive fluid is supplied to the HP fluid inlet 1 from an available source such as the recycle/discharge line of the main gas compression system (a standard component of an oil and gas production installation). The HP motive fluid could also be HP gas from HP wells or

HP liquids, including HP liquids supplied by mechanical pumps. The HP motive fluid is supplied to the jet pump 6 via the HP flow line 26 and the first HP flow control valve 2. The LP fluid (mainly LP gas) supplied from the LP fluid inlet 18 is usually passed through a dedicated knock-out pot 17, which removes any liquids carried with the LP gas. This knock out pot 17 is only needed if the LP fluid is expected to contain liquids comprising more than about 1% or 2% by volume of LP fluid at the operating pressure and temperature. The separated LP gas then passes through the first LP flow line 10 and often a gas flow meter 12, and in the case of conventional flaring passes through LP outlet line 9 and the first LP flow control valve 4 to the flare system (not shown) for burning. The collected liquids may be drained from the knock-out pot 17 through the drain valve 15 and drain line 16.

Now under an embodiment of the present invention, the flow of LP gas is diverted to the second LP flow line 20 and through the second LP flow control valve 3 and is fed to the LP jet pump inlet 6 a of the jet pump 6.

As soon as the flow of LP gas is detected by the flow meter 12, the control system comprising the control unit 28 and the control lines 29 sends control signals to the LP flow control valves 3,4 whereby the second LP flow control valve 3 opens, followed by the first LP flow control valve 4 closing. Substantially simultaneously, control system causes the first HP flow control valve 2 to open to feed HP gas to the HP inlet 6 b of the jet pump 6. Pressure transmitter 11 measures the pressure of the LP gas in the second LP flow line 20, under influence of the pressure drop generated by operation of the jet pump 6. This pressure reading is transmitted to the control unit 28, which adjusts the setting of the first HP flow control valve 2 to control the amount of HP gas delivered to the jet pump 6, according to the detected LP gas flow rate.

In this case if the LP gas flow rate is at its maximum, the first HP flow control valve 2 opens fully. If the LP gas is at its minimum rate then the first HP flow control valve 2 throttles to reduce the HP gas flow rate to ensure that the LP gas pressure generated by the jet pump 6 is maintained at close to 1 bar or any desired set point despite any fluctuations in the LP gas flow rate. The first LP flow control valve 4 remains closed but is operational for safety reasons and in cases where for some reason the jet pump 6 in unable to handle the flow of LP gas (or all of it), control valve 4 opens to flare some of the gas as soon as the pressure transmitter 11 shows the pressure has reached a set point above the normal accepted pressure. This may also happen in emergency cases when the pressure relief valve of an upstream process system opens and a much larger volume of gas is released, which is beyond the capacity of the supplied jet pump. The first LP flow valve 4 is also equipped with a bypass line 24 provided with a collapsing safety plate (as an example), which opens if the gas pressure detected by pressure detector 11 reaches a set maximum limit because of the failure of valve 4 to open.

The system is further protected by non-return valve 19 a in the jet pump discharge line 8 and non-return valve 19 b on the LP inlet line of the jet pump. Isolation valves 14 a, 14 b and 13 are used only for maintenance purposes when the jet pump system needs to be isolated for maintenance or change out of its internals if needed.

The operation of the control valves 2, 3, 4 will be controlled fully automatically via the control unit 28, which may for example be a SCADA (supervisory control and data acquisition) industrial control system, which monitors the said pressures and via a PLC (programmable logic controller) sends signals to the control valves 2, 3, 4 for their timely operation.

The system can be further protected by return flow line 7 and return flow control valve 5. This return flow control valve 5 opens if under low to no LP flow rate the first HP flow control valve 2 fails to close for any reason. When the return flow control valve 5 opens, return flow line 7 re-circulates some gas from the discharge line 8 of the jet pump, and this gas is fed via the second LP inlet line 20 to the LP inlet 6 a of the jet pump 6, so that the pressure of gas from the first LP flow line 10 does not drop below a specified value.

To avoid too many control operations of valves opening and closing, and to meet the need to capture flare/vent gas under unexpected conditions, the jet pump 6 can be run continuously. In such case, the above mentioned re-cycle flow through return line 7 could be used to avoid pulling pressure below atmospheric when no flare/vent gas is present. Also a neutral purge gas from line 23 can be introduced under jet pump suction. The flow direction meter 21 with Oxygen detector 22 in the first LP flow line downstream of the first connection point 25 can be used to indicate flow reserve and air being pulled-in in the absence of flare, thus allowing appropriate actions to be taken. The Oxygen detector 22 can detect oxygen if valve 4 opens erroneously and the system starts taking air from the flare system. In such case, the above mention control valves 2, 3, 4, 5 can be activated when required to prevent sub atmospheric low pressures and the intake of unwanted air.

If a suitable HP gas supply is not available, a mechanical pump may be used to supply the required HP motive liquid to the HP jet pump inlet 6 b. This HP liquid, which may be oil or water, then acts as the motive flow. The pressure boosting system may for example be similar to that described in GB 2450565A, the content of which is incorporated by reference herein. 

1. An apparatus for controlling the flow of a fluid, the apparatus including: a LP fluid inlet connected to a supply of LP fluid, a LP fluid outlet connected to a flare system for flaring LP fluid, a first LP flow line connecting the LP fluid inlet to the LP fluid outlet, a jet pump having a LP jet pump inlet, a HP jet pump inlet and a MP jet pump outlet, a second LP flow line having a first end connected to the first LP flow line at a first connection point between the LP fluid inlet and the LP fluid outlet and a second end connected to the LP jet pump inlet, a HP fluid inlet connected to a supply of HP fluid, a HP flow line having a first end connected to the HP fluid inlet and a second end connected to the HP jet pump inlet, a jet pump discharge line having a first end connected to the MP jet pump outlet and a second end connected to a MP fluid outlet; a first LP flow control valve in the first LP flow line between the first connection point and the LP fluid outlet, a second LP flow control valve in the second LP flow line between the first connection point and the LP jet pump inlet, a flow meter configured to detect the flow of LP fluid in the first LP flow line between the LP fluid inlet and the first connection point, and a control system connected to the flow meter and to the first and second LP flow control valves, wherein the control system is configured to control operation of the first and second LP flow control valves according to signals received from the flow meter.
 2. An apparatus according to claim 1, wherein the control system is configured such hat when the flow meter detects a flow of LP fluid, the control system closes the first LP flow control valve and opens the second LP flow control valve so that the LP fluid is directed into the LP jet pump inlet.
 3. An apparatus according to claim 2, including a first HP flow control valve in the HP flow line, said first HP flow control valve being connected to the control system, wherein the control system is configured such that when the flow meter detects a flow of LP fluid the control system opens the first HP flow control valve allowing HP fluid to flow from the HP fluid inlet into the HP jet pump inlet.
 4. An apparatus according to claim 3, including a pressure detector for detecting the pressure of fluid in the second LP flow line, said pressure detector being connected to the control system.
 5. An apparatus according to claim 4, wherein the control system is configured to adjust the first HP flow control valve according to the detected pressure of fluid in the second LP flow line, so as to control the flow rate of HP fluid into the HP jet pump inlet.
 6. An apparatus according to claim 4, wherein the control system is configured such that when the pressure detector detects a pressure in the second LP flow line that is higher than a predetermined value, the control system opens the first LP control valve so that at least some LP fluid is directed from the LP gas inlet towards the LP gas outlet.
 7. An apparatus according to claim 4, including: a return flow line having a first end connected to the jet pump discharge line and a second end connected to the second LP flow line between the second LP flow control valve and the LP jet pump inlet, and a return flow control valve in the return flow line, said control system being connected to the return flow control valve, wherein the control system is configured to control operation of the return flow control valve according to signals received from the pressure detector indicating the pressure of fluid in the second LP flow line.
 8. An apparatus according to claim 1, wherein the control system is switchable to an alternative operational mode, wherein the control system opens the first LP flow control valve and closes the second LP flow control valve so that the LP fluid is directed towards the flare system.
 9. An apparatus according to claim 1, including a knock-out pot in the first LP flow line between the LP fluid inlet and the first connection point.
 10. An apparatus according to claim 1, including a by-pass line connected around the first LP flow control valve and a pressure release device provided within the by-pass line.
 11. An apparatus according to claim 1, including an oxygen detector in the first LP flow line, wherein the control system is configured to close the second LP flow control valve if oxygen is detected in the first LP flow line.
 12. An apparatus according to claim 1, including a plurality of jet pumps connected in parallel.
 13. A method for controlling the flow of a fluid in an apparatus including a LP fluid inlet connected to a supply of LP fluid, a LP fluid outlet connected to a flare system for flaring LP fluid, a first LP flow line connecting the LP fluid inlet to the LP fluid outlet, a jet pump having a LP jet pump inlet, a HP jet pump inlet and a MP jet pump outlet, a second LP flow line having a first end connected to the first LP flow line at a first connection point between the LP fluid inlet and the LP fluid outlet and a second end connected to the LP jet pump inlet, a HP fluid inlet connected to a supply of HP fluid, a HP flow line having a first end connected to the HP fluid inlet and a second end connected to the HP jet pump inlet, a jet pump discharge line having a first end connected to the MP jet pump outlet and a second end connected to a MP fluid outlet; a first LP flow control valve in the first LP flow line between the first connection point and the LP fluid outlet, a second LP flow control valve in the second LP flow line between the first connection point and the LP jet pump inlet, a flow meter configured to detect the flow of LP fluid in the first LP flow line between the LP fluid inlet and the first connection point, and a control system connected to the flow meter and to the first and second LP flow control valves, wherein the control system is configured to control operation of the first and second LP flow control valves according to signals received from the flow meter, the method comprising: supplying LP fluid to the LP fluid inlet, supplying HP fluid to the HP fluid inlet, detecting the flow of LP fluid in the first LP flow line between the LP fluid inlet and the first connection point, and controlling operation of the first and second LP flow control valves according to signals received from the flow meter.
 14. A method according to claim 13, comprising closing the first LP flow control valve and opening the second LP flow control valve when the flow meter detects a flow of LP fluid, to direct LP fluid into the LP jet pump inlet.
 15. A method according to claim 13, comprising opening the first HP flow control valve when the flow meter detects a flow of LP fluid, to allow HP fluid to flow from the HP fluid inlet into the HP jet pump inlet.
 16. A method according to claim 13, comprising detecting the pressure of fluid in the second LP flow line.
 17. A method to claim 14, including controlling the flow rate of HP fluid according to the detected pressure of fluid in the second LP flow line.
 18. A method according to claim 14, including opening the first LP control valve to direct at least some LP fluid from the LP gas inlet towards the LP gas outlet when the pressure detector detects a pressure in the second LP flow line that is higher than a predetermined value.
 19. A method according to claim 14, including controlling operation of the return flow control valve according to signals received from the pressure detector.
 20. A method according to claim 14, wherein the jet pump is operated continuously.
 21. A method according to claim 14, including selecting an alternative operational mode comprising opening the first LP flow control valve and closing the second LP flow control valve to direct LP fluid towards the LP fluid outlet. 